I could see them adding HP, yes you may decrease the amount of people from the UK going to the US, but either way you will do that.
Universal also can get a major boost, yes less people are going to Florida but how many more people will you get to go to the UK one, the amount of people able to afford the multiple thousand pound trip to Florida will be much less than the amount of people who can afford the couple hundred pounds to travel to and visit the UK park, not adding HP could be like shooting yourself in the foot.
a side note is also that the people who may have travelled to the US are also now not going to Disney, this means they have more money for universal as they aren't spending the money on Disney, flights, etc.
WB also is an interesting one, I am curious to see how they would be impacted, currently the tickets are £56, universal would probably be charging more for entrance so it could be seen as an alternative cheaper day out (although £56 seems very expensive) and it also has the props, scenes, the actual train used in filming (removed from in service *grumble*) and much more, making it more of a harry potter focused thing, I think it could be improved as potter fans come to the UK to experience platform 9 and 3/4 (maybe going on the Hogwarts express train thing they run), then go to the studio tour, then go to universal.